1. Field of the Invention
The present invention relates to the construction of the stator of a progressive cavity pump or motor, and a method of assembly therefor.
2. Background of the Technology
Progressive Cavity (PC) pumps and motors have been used for years. The principle of operation of PC pumps was first described by Rene Moineau in his 1931 thesis and has since been known as the Moineau principle. Typically a PC pump or motor consists of a rigid helical rotor in a double-lead helical cavity stator. The differences in the leads between the rotor and the stator form cavities that progress axially from one end of the stator to the other as the rotor turns, moving the fluid through the pump or motor.
The stator is conventionally made of an elastomeric or plastic material thread housed into a typically metallic, rigid, sleeve-shaped outer tube. The helical profile of the stator thread is typically formed by injection moulding the elastomeric or plastic material into the outer tube around a core. During normal operation of a PC pump, the rotor operates in tight contact with the stator thread, generating a high torsional force between the rotor and the stator. Accordingly, a tight bond is required between the stator thread and the outer tube in order to obtain a torsionally rigid structure. Additionally, a tight bond is required in order to provide a fluid seal between the stator thread and the outer tube.
The stator thread may simply be moulded inside a bare tube and be attached to the outer tube by bonding it to the inside of the outer tube using adhesives. Bonding using an adhesive, however, limits the use of the stator to an operational temperature and chemical environment required by the adhesive. Operating conditions beyond the temperature and chemical environment required may lead to the breakdown of the bond causing the stator to detach from the outer tube.
It is well known, in many situations where rubber or elastomer is to be connected to a rigid body, to use not only bonding, whether by natural adhesion of the elastomer or by assisted adhesion with a chemical bonding agent, but also to provide a mechanical connection. This is potentially achieved by providing re-entrants in the body to which the elastomer is being connected to achieve a mechanical interlock. It is also achieved by providing a cage or the like fixed to the body by some means, and through which cage the elastomer is moulded, also achieving a mechanical interlock, albeit indirectly through the cage, with the body. Such re-entrants could be provided in the bore of the stator tube in the case of PC pumps and motors but, of course, providing such re-entrants is problematic. Indeed, the problem would be extreme with long stators such as employed in mud motors used in the oil industry that may call for stators of up to eight or more meters in length. Thus the provision of a cage or the like is more suitable. Recently, there have been several designs of stators that bond mechanically to the outer tube (thereby eliminating the need for an adhesive) and U.S. Pat. Nos. 7,407,372, 7,316,548 and 7,329,106 disclose different methods of mechanically bonding stator threads to inner perforated stator tubes that are connected by welding to the outer stator tube. The inner stator tubes incorporate radial apertures and are placed and welded to the outer tube prior to the injection moulding of the stator material.
Such stators are assembled by disposing an appropriate anchor element within the outer tube and permanently fixing the two together, typically using welded fixings at various locations along the length of tubes, prior to injection moulding the stator material. Furthermore, a fluid seal between the stator material and the outer stator tube is always necessary and, where no special adhesion is employed, another means is required of achieving it. In the patents just mentioned, this is, for example, achieved using sealing rings that compress the stator material following injection moulding. However, it is also suggested to form re-entrant grooves in the inner stator tube and sealing the elastomeric thread to the inner tube.
Manufacturing stators such as the ones described above, requires the use of complex manufacturing processes, customised for the production of each individual stator. Also, it is difficult, particularly in long tubes, to insert mechanical bonding tube cages inside stator tube housings and have them in any sort of a close fit, and so that they fit concentrically in the stator housing. Either they will snag on some inequality of the bore diameter of the housing and jam during insertion (this is a particular problem on longer stators) or they are too loose to form a stable bonding anchor for the stator thread material. It is an object of the present invention to provide an arrangement which addresses these issues. Indeed, it is an aim of embodiments of the invention to provide a stator that is simpler to manufacture.